LocalStorage: update LevelDB block cache to better reflect reality.

Previously, LocalStorage used the shared web block cache, but explicitly
disabled caching on almost every read call. This was deceptive for two
reasons:

* the cache is provided but not actually used
* the cache is called "shared" but it would only be shared with other
  LevelDB-backed web storage APIs that run in the storage service
  process, which excludes the other two that might share it, i.e.
  IndexedDB and push notifications.

This change should have essentially zero behavior impact as it just
changes the way in which caching is disabled.

Bug: none
Change-Id: I8e1b7d845f1bdad231dbf4b71c74a4138528be4e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5512927
Reviewed-by: Ayu Ishii <ayui@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1296206}